Does P0420 always mean I need a new catalyst?

No. Sensor faults and upstream engine issues can produce P0420 without the catalyst itself being at fault. A diagnostic session is needed to confirm the root cause before any parts are ordered.

Will P0420 cause an MOT failure?

An exhaust emissions fault and a dashboard warning light are both MOT failure points. A properly functioning catalyst is required to pass the emissions check.

Can a damaged catalyst be cleaned rather than replaced?

In some cases a catalyst that is soot-loaded but not physically degraded can recover after correct operation. A catalyst that is structurally damaged or contaminated requires replacement.

How is a removed or bypassed catalyst identified?

Through exhaust temperature behaviour and sensor plausibility. A gutted or bypassed catalyst shows distinctive temperature patterns and sensor readings that differ from a degraded but present unit.

Mercedes P0420: Catalyst System Efficiency Below Threshold

P0420 is stored when the ECU determines that the catalytic converter is not reducing harmful emissions to the required level. On Mercedes diesel vans it is often associated with the oxidation catalyst upstream of the DPF. It is a code that appears straightforward but has several possible causes beyond a failed catalyst.

What P0420 means

The catalyst efficiency is monitored by comparing exhaust gas composition upstream and downstream of the catalyst using temperature sensors, NOx sensors on equipped models, and the oxygen or lambda sensor where fitted. When the ECU calculates that the conversion efficiency is below threshold, P0420 is stored. This can reflect a genuinely degraded catalyst, a sensor giving incorrect readings, or an upstream fault that is preventing the catalyst from reaching its correct operating temperature.

Common causes on Mercedes diesel vans

A degraded or physically damaged oxidation catalyst is the most direct cause. However P0420 can also be triggered by: a faulty exhaust temperature sensor upstream or downstream of the catalyst giving incorrect efficiency readings, a failed or contaminated NOx sensor on SCR-equipped models creating a false efficiency assessment, an engine fault causing excessive unburned fuel to enter the exhaust and overheat or contaminate the catalyst, and a catalyst that has been physically removed or bypassed. This last cause is identifiable during a diagnostic session and is not something we will advise on or facilitate.

What a diagnostic session looks at

Exhaust temperature sensor plausibility upstream and downstream is checked first. If sensor readings are implausible relative to engine load and conditions, the sensor rather than the catalyst may be at fault. On SCR-equipped models the NOx sensor data is reviewed for correct function. Engine fuelling and combustion health is checked to rule out an upstream fault contributing to catalyst degradation. Physical inspection of the catalyst housing where accessible. If all sensors are plausible and no upstream fault is present, the catalyst itself is the most likely cause.

Mercedes P0420: Catalyst System Efficiency Below Threshold

What P0420 means

Common causes on Mercedes diesel vans

What a diagnostic session looks at

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